The present invention relates to a method for separating light additive gas at the end of a separating nozzle cascade and/or at the location of the change in stage size in a separating nozzle cascade, the separating nozzle cascade operating with a mixture of gaseous or vaporous substances to be separated and a light additive gas.
In separating nozzle processes for the separation of gaseous or vaporous mixtures, particularly isotope mixtures, a light additive gas is used in molar excess so as to improve the economy of the process. In this type of arrangement, the separating nozzle process has gained special significance in connection with the enrichment of the light uranium isotope .sup.235 U for nuclear reactors. The mixture to be separated then comprises the isotope molecules .sup.235 UF.sub.6 and .sup.238 UF.sub.6, while the light additive gas is usually hydrogen or helium. To simplify matters, the process according to the present invention will be explained below for such an example in which hydrogen (H.sub.2) is assumed to be the light additive gas. The process, however, can also be used with technical and economical success for other substance mixtures to be separated and other light additive gases.
In order to realize the .sup.235 U concentration of about 3.2% required for light water nuclear power plants, several hundred separating nozzle stages, with each stage generally including a separating nozzle and a compressor, must be connected in series in a so-called isotope separating cascade. In the best cascade connection for isotope separation, the undesirable demixing of UF.sub.6 and H.sub.2 which occurs along the isotope separating cascade is automatically reversed when the various partial streams are combined. However, at the head of the isotope separating cascade and at points where stage sizes change in the cascade, there appears an extra stream of hydrogen which contains UF.sub.6 and which must there be separated in a so-called UF.sub.6 separation system and returned to the isotope separating cascade at suitable locations. Since the mixing ratios of the uranium isotopes differ greatly at the separation and input locations, the recycled hydrogen must contain practically no UF.sub.6. In an industrial system, a residual content of a few ppm UF.sub.6 can already cause a production loss of enriched uranium in the order of magnitude of several percent.
The separation of UF.sub.6 and H.sub.2 in a UF.sub.6 separation system in the form of a gas separating cascade comprising 8 to 10 separating nozzle stages has been tried and found to be too expensive, as reported in KfK Report 1437, Kernforschungszentrum Karlsruhe, July, 1971. Accordingly, a combination of only one separating nozzle stage for preliminary separation (preliminary separation stage) and a system of switchable low temperature countercurrent separators (low temperature separators) has been provided as a UF.sub.6 separation system. Clogging of the low temperature separators by frozen, solid UF.sub.6 is to be prevented in this system by a computer controlled supply of coolant. See German Patent No. 2,654,249.2 and corresponding U.S. Pat. No. 4,181,508. In a more recent publication, the coolant input is controlled automatically by means of the pressure drop at the low temperature separator. See KFK Report 3196, Kernforschungszentrum Karlsruhe, July, 1981.
In the prior art, the preliminary separating stage continuously returns approximately 70% of the UF.sub.6 contained in the extra H.sub.2 stream to the head of the isotope separating cascade or to the point where stage sizes change in the isotope separating cascade. The remaining 30% of the UF.sub.6 are frozen in the low temperature separators and develop discontinuously during the heating phases employed in the operation of the low temperature separators. The UF.sub.6 coming from the low temperature separators must therefore be intermediately stored in a UF.sub.6 buffer and fed back in regulated amounts into the isotope separating cascade. At the same time, the "product stream" is obtained at the head of the UF.sub.6 separation system. In the ideal state, there is no fluctuation in the buffer influx and no fluctuation in the buffer outflux, and this product stream corresponds precisely to the difference between the buffer influx and the buffer outflux. In the prior art, these buffer streams are greater, by a factor of 50 to 100, than the product stream. Therefore, relatively small fluctuations in these buffer streams, unless they are correlated, lead to relatively great changes in the difference between buffer influx and buffer outflux. This difference represents the stream effectively discharged from the cascade at the cascade head. Due to the large ratio of the buffer streams to the ideal product stream, even small fluctuations in these buffer streams (e.g. 1%), as they can hardly be prevented in practice, already lead to relatively great fluctuations in the ratio between the stream effectively discharged from the cascade and the ideal product stream (e.g. 50 to 100%). Experience has shown that such fluctuations can lead to considerable losses in the average production output of the system because of the isotope mixing connected therewith.